1. Field of the Invention
The present invention relates to a transmitter power varying device for varying the transmitter power over a wide dynamic range, and particularly relates to a transmitter power varying device for use in, for instance, a mobile telephone set of a cellular telephone system in which the power consumption is an important item to be addressed.
2. Description of the Related Art
In recent years, for more efficient reuse of frequency bands in cellular telephone systems, a mobile communications system according to the CDMA (code division multiple access) scheme using a spread spectrum technique is now entering a practice stage. In this system, to solve what is called a near-far problem, it is required to vary, quickly with high accuracy, the transmitter power over a range that is at least as wide as a little more than 70 dB. It has been reported that this system has an advantage that the average output power level is much smaller than conventional analog cellular telephone systems. On the other hand, since this system employs digital modulation, linear amplification needs to be performed. Therefore, the operating point is set in a region of a class-A amplifier which region assures superior linearity.
FIG. 12 shows an example of a conventional transmitter power varying device. An IF signal that is output from an IF signal oscillator 1 is modulated by an IF signal modulator 2 and then subjected to transmitter power varying control by an IF band variable gain amplifier 3. An output of the amplifier 3 is mixed, by an up-converter mixer 4, with a local frequency signal of a local signal oscillator 5. An output of the mixer 4 is amplified by a RF amplifier 6, then power-amplified by a power amplifier 7, and finally transmitted from an antenna 9 via an antenna duplexer 8. As such, in the transmitter power varying control of the conventional transmitter power varying device, the IF stage covers the entire variable range.
However, in the conventional transmitter power varying device, since the power amplifier performs a class-A (or class-AB) operation as described above, the consumption current cannot be reduced though the transmitter power can be made much smaller than in an analog-mode operation; that is, the above transmitter power varying device operates with very low power conversion efficiency. Further, since the conventional transmitter power varying device is required to cover a dynamic range of more than 80 dB, there remain difficult problems in connection with the characteristics of the variable gain amplifier, such as the linearity of the control characteristic, temperature dependence, distortion characteristic, noise characteristic, and consumption current characteristic.